Typically, modern electrical products incorporate printed circuit assemblies (PCAs), such as printed circuit boards (PCBs). The range of products is immense, including cell phones, laptops televisions, MP3 players, game consoles, personal data assistant and aircraft components, to name just a few.
The printed circuits within these products interconnect a variety of circuit components, such as diodes, transistors, resistors, integrated circuits and the like. Fabricated as individual components, each generally has one or more legs or pins (commonly referred to as leads). The individual components are brought into useful harmony by a circuit board that provides electrical traces to and from different components as well as areas that facilitate the permanent mounting of components upon the board.
Due to fabrication complexity of many products, the PCAs are assembled in stages. A given PCA and at least some of the components thereon may therefore be subjected to repeated processing steps. As such, the components frequently require monitoring and testing during the fabrication process to insure that the ultimate device is functional. If an uncorrectable defect is detected early, additional fabrication costs may be saved by halting further assembly of the defective product.
Electrical test probes are used to provide electrical connections between PCA components and testing instruments. An electrical test probe generally consists of an electrically conductive probing tip joined to an electrically conductive shaft that is in turn connected to a test fixture, which attempts to align the probe to a specific component.
Generally speaking, the components are attached to the PCA by solder. Economic and environmental factors have effected a change in the solder process from lead based solder to lead free solder. The use of lead free solder imposes additional fabrication issues upon the assembly and testing process. For through-hole-technology (THT) components, the process and costs of wave soldering can be eliminated by assembling these to the board using through-hole reflow (THR). THR is a way to mount THT components simultaneously with the surface-mount-technology (SMT) components. Typically, the solder is applied in a paste form with the use of a stencil to the circuit board. Components are then pressed into the solder paste, and/or into holes in the board along with solder paste. The board is then heated to the solder melt temperature to reflow the solder such that it wets a pad surface and/or flows about the pins of a component to be joined to the board. In addition to the solder metal, the solder paste also contains a combination of chemicals called flux, which help keep the solder in a paste form, act as adhesive so the paste sticks to the pads and pins, thereby holding the components on the board before being reflowed, and clean the metal of pads and pins in order to achieve a good solder joint. The reflowing process releases the flux components of the paste and leaves flux residue on the board and solderjoints. The flux residue is a combination of non-conductive materials.
Holes in the board are frequently used to mount components and/or provide board interconnections. When the reflowed solder flows into these holes it may partially or completely fill them. Flux material also will flow into the hole and gather on top of the reflowed solder. The flux material may lie below the pad of the hole, be flush with the it or flood over it.
When the hole and/or its surrounding pad are the target of a test probe, the flux residue may prevent a reliable and repeatable electrical connection between the pad and the target when urged with each other. Also, a certain amount of force is generally used when the test probe tip is urged into the solder. If too much force is applied, this may break solder joints, components or the board itself. If too little force is applied the probe may not make sufficient contact with the solder and a valid component may be judged to be defective. Thus, a low force that repeatably makes good electrical contact between a test probe and its target is desirable.
Most conventional test probe tips are generally in the shape of a cone or other shape that narrows to a point. Such a point in line with the probe's longitudinal axis permits a concentration of force in line with that axis, and thus also limits probe wear. With respect to a through hole filled with solder having concave meniscuses in turn filled by flux residue pooled therein, the conventional probes point targets the deepest portion of the flux pool. Attempts to contact the solder may thus be frustrated, and testing may fail despite the node actually being properly functional.
Probe tips in the shapes of cups, crowns and radial stars with three or more tips for alignment over mounded solder elements also exist. However, as the number of contact points increases, so too does the surface area of contact. More specifically, as the points of contact increase, the concentration of applied force transferred to each point decreases.
This can be illustrated by the example of a man on snow shoes. The man may walk across a soft snow because the snow shoes distribute his weight upon the snow across a large surface area. In a more specific example, a force magnitude of 12 units (arbitrary) applied by a single point to a surface will transfer a force magnitude of 12. The same force applied by three points sees each point apply a force magnitude of only 4—a third of the total force (12÷3=4). In other words, the contact force applied by the plunger is divided by the number of contact points, resulting in a lower contact force per tip. Materials limit how small the contact surface area of each tip can be made. Thus the pressure applied by a single tip probe will be three times higher than that applied by each tip of a three tip probe—assuming all tips have equivalent contact surface areas.
Thus, the multiplicity of points of contact from start tips, crown tips or the like may further frustrate the attempt to achieve a proper electrical contact between the probe tip and the solder. Single flat blade probe tips are likewise also frustrated by the presence of flux, as they provide a large surface area for contact and thus result in lower contact pressure per unit of contact area.
In addition, cupped tips and multi point tips may easily be fowled by flux material. As the probe tip attempts to reach the solder below the flux material, the flux material is compacted into the cup and/or between and about the multiple tips. Such material may collect to such a point where the probe tips are simply unable to make electrical contact, with even clean test locations.
In short, single point tips are not well suited for probing through holes clogged or capped by flux material as the single point tip tends to be aligned for center of the hole where the flux material is most thick. Flat blade probe tips and tips with three or more tips result in force distribution over an increased surface area. Multi-point tips, which may avoid the thick portions of flux material, have less force to penetrate through the flux residue and are easily fouled by flux collecting at the probe tip, and are therefore unable to make repeatable and reliable electrical contact.
Consequently, the necessary electrical contact between the probe and the solder is not achieved in all situations and the testing system may wrongly evaluate a healthy board and/or component as defective, due simply to the contact failure. Also, bad contact may lead to incorrectly passing a bad board. Such incorrect evaluations are costly, either due to costly troubleshooting involved, good product becoming scrapped or profitability being impacted by bad product becoming deployed and in turn necessitating costly customer support under warranty.
Hence, there is a need for a device that overcomes one or more of the drawbacks identified above.